Doctor of Philosophy, University of Toledo, 2016, Physics

Spectroscopic ellipsometry (SE) is a non-invasive optical probe that is capable of accurately and precisely measuring the structure of thin films, such as their thicknesses and void volume fractions, and in addition their optical properties, typically defined by the index of refraction and extinction coefficient spectra. Because multichannel detection systems integrated into SE instrumentation have been available for some time now, the data acquisition time possible for complete SE spectra has been reduced significantly. As a result, real time spectroscopic ellipsometry (RTSE) has become feasible for monitoring thin film nucleation and growth during the deposition of thin films as well as during their removal in processes of thin film etching. Also because of the reduced acquisition time, mapping SE is possible by mounting an SE instrument with a multichannel detector onto a mechanical translation stage. Such an SE system is capable of mapping the thin film structure and its optical properties over the substrate area, and thereby evaluating the spatial uniformity of the component layers. In thin film photovoltaics, such structural and optical property measurements mapped over the substrate area can be applied to guide device optimization by correlating small area device performance with the associated local properties. In this thesis, a detailed ex-situ SE study of hydrogenated amorphous silicon (a Si:H) thin films and solar cells prepared by plasma enhanced chemical vapor deposition (PECVD) has been presented. An SE analysis procedure with step-by-step error minimization has been applied to obtain accurate measures of the structural and optical properties of the component layers of the solar cells. Growth evolution diagrams were developed as functions of the deposition parameters in PECVD for both p-type and n-type layers to characterize the regimes of accumulated thickness over which a-Si:H, hydrogenated nanocrystalline silicon (nc-Si:H) and mixed phase (a+n (open full item for complete abstract)

Committee: Robert Collins (Advisor) Subjects: Materials Science; Physics

Doctor of Philosophy, University of Toledo, 2024, Physics

Perovskites materials with ABX3 (A: monovalent cation; B: divalent cation; X: anion) structure manifest interesting properties and applications in materials science and photovoltaics (PV) as thin films and solar cells. Optimization of their application requires a profound understanding of optoelectronic and structural properties of these materials and correlation with device performance. This dissertation presents the wide range of spectroscopic ellipsometry studies performed on organic-inorganic metal halide-based perovskites films and solar cells, LaMnO3 complex oxide perovskite epitaxial films, and NiOx films as hole transport materials for perovskite-based PV devices. Degradation of organic-inorganic metal halide-based perovskites films with exposure to relative humidity and temperature is studied by continuously tracking changes in structural and optical properties with time. Narrow bandgap perovskite devices incorporating different hole transport materials are aged in ambient air to track degradation and quantify losses towards front and back contact interfaces of the devices from external quantum efficiency simulations. Optical properties of LaMnO3 complex oxide perovskite are reported and interpreted. Optical properties and morphology of NiOx films prepared using nanoparticle, spray coating, and sputtering methods are determined to optimize preparation methods of NiOx for specific application. Organic-inorganic metal halide ABX3 perovskites (A cation: methylammonium-MA, formamidinium-FA, cesium-Cs, rubidium-Rb; B cation: lead-Pb, tin-Sn; X anion: iodine-I, bromine-Br, chlorine-Cl) gained great interest recently in optoelectronic and photovoltaic applications due to their unique properties including high absorption coefficient, long charge carrier diffusion length, low-cost solution techniques for their fabrication, and high efficiency of the solar cell devices based on these materials. Aside from their great advantages, stability is a major challenge for (open full item for complete abstract)

Committee: Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Michael C. Young (Committee Member); Aniruddha Ray (Committee Member); Yanfa Yan (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2023, Physics

Low-cost thin film absorber layer materials with high absorption coefficients (> 105 cm-1 in visible spectral range) and bandgap close to the ideal value for efficient photovoltaic conversion efficiency are leading candidates for thin film photovoltaic (PV) applications. This dissertation discusses the fabrication and optical and microstructural properties of magnetron-sputtered glancing angle deposited CdTe thin film absorber layer material and its application as an interlayer in CdS/CdTe solar cells. In addition, optoelectronic properties of non-toxic and earth-abundant absorber layer material, antimony selenide (Sb2Se3), and optimization of polycrystalline VO2 fabrication from amorphous vanadium oxide (VOx) film along with its optical properties have been discussed. Sb2Se3 is a promising candidate as an absorber layer material in PV applications. I have performed optical property characterization of thin film Sb2Se3 and identified electronic losses when used in a PV device. The indirect bandgap, direct bandgap, and Urbach energy have been determined to be 1.12 eV, 1.17 eV, and 21.1 meV, respectively using photothermal deflection spectroscopy. Optical properties of Sb2Se3 in the form of complex dielectric function (ε = ε1 + iε2) spectra in 0.75 to 4 eV spectral range is determined using spectroscopic ellipsometry. The line shape of ε is obtained using a parametric model which incorporates an Urbach tail, a band edge function, and five critical point oscillators. The optical property spectra in ε and structural parameters in terms of the thickness of solar cell layer components are used as input parameters for external quantum efficiency (EQE) simulation to investigate the electronic and optical losses in Sb2Se3-based solar cells. A carrier collection length of ~ 400 nm and a ~97 % carrier collection probability near the heterojunction in the Sb2Se3 solar cell are identified by comparing experimental and simulated EQE. Next, I describe deposition and characterizati (open full item for complete abstract)

Committee: Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Yanfa Yan (Committee Member); Song Cheng (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2022, Physics

This dissertation represents a collection of related studies that employ the various capabilities of spectroscopic ellipsometry (SE) to characterize and gain insights into the properties of the materials of relevance to advanced cadmium telluride (CdTe) photovoltaic technology. The structural, optical, and electrical properties of the component layers of the CdTe solar cell have been investigated using different SE data collection modes and analysis techniques. The component layers of the CdTe solar cell are deposited on soda lime and TEC-15 glass substrates in the superstrate configuration, i.e., with the solar irradiance entering through the glass substrate. The key layers include a transparent conducting oxide front contact, typically pyrolytic fluorine-doped tin oxide (SnO2:F); a high resistivity transparent layer (HRT) of either pyrolytic SnO2, sputtered MgxZn1 xO (MZO), or both; semiconductor layers of either n-type cadmium sulfide (CdS), cadmium selenide (CdSe), or both; p-type CdTe; a p+ back contact interlayer typically copper based; and a metallic conducting back contact layer, such as gold. In this research, SE-deduced component layer properties have been correlated with the CdTe device performance parameters. Applying various SE capabilities not only identifies the correlations between the material properties and solar cell performance but also enables optimization of the preparation conditions (e.g., substrate temperature) and resulting properties (e.g., thickness) of the CdTe device components. First, we have employed UV-VIS SE and ex-situ mapping SE results to correlate between the CdSe optical and structural properties with the CdTe solar cell performance. The effects of varying CdSe layer thickness on the CdTe solar cell performance have been studied, focusing on the TEC-15/HRT/CdSe/CdTe/Cu/Au cell structure. A set of four 6.5 cm x 6.5 cm TEC-15/HRT structures were coated with different nominal thicknesses of CdSe for incorporation within the (open full item for complete abstract)

Committee: Robert Collins Dr. (Advisor); Jone Bjorkman Dr. (Committee Member); Stephen O'Leary Dr. (Committee Member); Randall Ellingson Dr. (Committee Member); Nikolas Podraza Dr. (Committee Member) Subjects: Meteorology; Optics; Physics

Doctor of Philosophy, University of Akron, 2020, Polymer Engineering

The nature of interfacial interactions can significantly alter the physical and mechanical properties of thin polymer films. Glassy films are particularly prone to changes in physical aging in thin films, which can significantly alter the properties in applications. A glassy polymer film is initially inherently in a non-equilibrium state, but the chains will slowly relax towards equilibrium below the glass transition temperature (Tg). This aging can be assessed with a variety of properties from thermodynamic to volumetric to dynamic. In this work, the physical aging of a promising copolymer for membrane separations, butylnorbornene-ran-hydroxyhexafluoroisopropyl norbornene (BuNB-ran-HFANB), in thin films was systematically investigated. The BuNB-ran-HFANB film thickness and aging temperature both impacted the aging rate of the films. We observed rapid densification of the BuNB-ran-HFANB polymer even when aged >200°C below Tg via ellipsometric measurements during isothermal aging process. However, for the thinnest polymer films, the film thickness increases instead of decreases during aging, which we attribute to an initial highly non-equilibrium state for the BuNB-ran-HFANB chains as a result of the rapid evaporation during the spin coating process. The substrate selection for these films impacted the observed physical aging of the BuNB-ran-HFANB films with bare silicon, 2nm polystyrene sulfonate (PSS), and 2nm poly(acrylic acid) (PAA) coatings investigated as substrates for the BuNB-ran-HFANB films. The unusual aging behavior of increasing film thickness was seen for the thinnest polymer films for all three substrates, but the onset for the increasing thickness during aging was substrate dependent (<250nm for PSS, <115nm PAA, <150nm silicon). To understand this behavior, the stress state after different aging times were determined via wrinkling experiments on the aged films. The residual stress in the thin films initially increased during aging, but the stress t (open full item for complete abstract)

Committee: Bryan Vogt (Advisor); Kevin Cavicchi (Advisor); Thein Kyu (Committee Chair); Abraham Joy (Committee Member); Jutta Luettmer-Strathmann (Committee Member) Subjects: Polymer Chemistry; Polymers

Doctor of Philosophy, University of Toledo, 2019, Physics

Thin film solar cells are promising candidates for generation of low cost and pollution-free energy. The materials used in these devices, mainly the active absorber layer, can be deposited in a variety of industry-friendly ways, so that the cost associated with manufacturing is generally lower than for competing technologies such as crystalline silicon. This dissertation will focus on the fabrication and characterization of nanocrystalline hydrogenated silicon (nc-Si:H) and polycrystalline cadmium telluride (CdTe) thin films by industrially scalable, non-toxic, and comparatively simple magnetron sputtering. The performance of the solar cells incorporating these films as an active absorber layers are discussed. In this work, spectroscopic ellipsometry is used as the primary tool for the characterization of optical and structural properties of thin films and bulk material. As a first case study, the anisotropic optical properties of single crystal strontium lanthanum aluminum oxide (SrLaAlO4) in the form of birefringence and dichroism is obtained from Mueller matrix ellipsometry. SrLaAlO4 exhibit uniaxial anisotropic optical properties and the indirect optical band gap of 2.74 eV. A parametric model consisting of parabolic band critical points (CPs) for electronic transitions and a gap function is used to describe the complex dielectric function spectra in both the ordinary and extra-ordinary directions. The modeling in this case study has applications to both nc-Si:H, an indirect band gap semiconductor, and CdTe which may exhibit microstructural anisotropy depending upon the deposition method. Fabrication and characterization of hydrogenated silicon (Si:H) thin films produced by reactive magnetron sputtering is the second case in this study. RTSE and a virtual interface analysis (VIA) are used to track the growth evolution of sputtered Si:H. From these studies, growth evolution diagrams depicting the nucleation of nanocrystallites from the amorphous phase and (open full item for complete abstract)

Committee: Nikolas Podraza (Committee Chair); Robert Collins (Committee Member); Yanfa Yan (Committee Member); Michael Cushing (Committee Member); Sylvain Marsillac (Committee Member) Subjects: Energy; Materials Science; Optics; Physics

Doctor of Philosophy, University of Toledo, 2019, Physics

Evaluation and understanding of optical properties are essential for the use and design of optoelectronic devices. This dissertation explains the evaluation of optical properties of component layers of the encapsulated photovoltaic (PV) module and uses them in device simulation focusing on thermal management. Sub-bandgap characterizations are not emphasized enough in the PV device design earlier. The examples discussed here range from ordinary glass used to cover solar cells to completed silicon (Si) wafer and thin film cadmium telluride (CdTe) solar cells. This study will focus on two key mechanisms for thermal management: radiation and sub-bandgap reflection. Commercial solar cells have light incident through the glass in solar wavelength range ~250- 2500 nm. This cover glass has an ability to re-radiate heat in the infrared (IR) region, thermal wavelength, from the device to keep the solar cells cool. In contrast to bare semiconductors, glass has a relatively high emissivity aiding in radiative cooling of solar modules. The directional thermal emissivity of solar cell cover glasses with differences in glass composition or manufacture and surface texture are evaluated using specular and specular+diffuse infrared reflectance at a different angle of incidences. Non-textured and textured glasses all exhibit similar emissivity at all angles of incidence regardless of composition and patterning. Both diffuse and specular reflectance must be included for textured glass at any angle of incidence and may be needed for planar glass at a high angle of incidences to determine emissivity accurately. Optical characterization of the semiconductor is important from the perspective of physics and application in devices. There are different features in the optical response related to different physical phenomena such as band to band electronic transitions, vibrational or phonon modes, and free carrier absorption. I have explored optical properties of an epitaxial indium pho (open full item for complete abstract)

Committee: Nikolas Podraza (Committee Chair); Robert Collins (Committee Member); Yanfa Yan (Committee Member); Sanjay Khare (Committee Member); Michael Deceglie (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2017, Physics

Spectroscopic ellipsometry (SE) is a powerful tool for non-destructive evaluation of thin films consisting of single layers or multilayers on substrates. For such thin films, SE can provide structural parameters and component material optical properties over a wide spectral range. Further analyses of these optical properties can provide additional information of interest on the physical and chemical properties of the materials. Installation of the SE instrument on a deposition chamber enables monitoring of the thin film during deposition. This experiment, referred to as in-situ real time SE (RTSE), enables high surface sensitivity, fast data acquisition, and non-invasive probing which lead to unique insights into the dynamics of film growth. In this dissertation research, RTSE was applied for analysis of the structural evolution of oxygenated CdS (CdS:O) films deposited on c-Si substrates using different [O2 ]/{[Ar] + [O2]} gas flow ratios. The analysis of RTSE data provides valuable information including the initial film growth mode (e.g. layer-by-layer or clustering), subsequent bulk layer and surface roughness thickness evolution, the growth rate in terms of effective thickness (or volume/area), and the final film optical properties. As an additional application of SE, ex situ through-the-glass mapping SE (TG-M-SE) has been used to study the structural properties and area uniformity of CdS/CdTe solar cells over large areas. The mapping results can be correlated with the efficiency of solar cells fabricated over the same area, exploiting inevitable non-uniformities in the process to identify the optimum structural parameters for highest efficiency solar cells. In a second component of this research, RTSE has proven to be very powerful for the development and optimization of thin film FeS2 deposited by a novel hybrid sputtering/co-evaporation method. This effort started with the sputtering of elemental Fe metal thin films and proceeded to the evaporation (open full item for complete abstract)

Committee: Robert W. Collins (Committee Chair); Nikolas J. Podraza (Committee Member); Randall J. Ellingson (Committee Member); Thomas J. Kvale (Committee Member); Stephen O’Leary (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2017, Physics

Spectroscopic ellipsometry (SE) is an important optical characterization technique, applicable to a wide variety of samples such as a single substrate material to a multilayer complex structured opto-electronic device. SE is always known to be an accurate and precise measurement of structural, optical, and electrical properties of material especially for a multilayer stack, including photovoltaic devices. This presentation demonstrates examples ranging from a gadolinium gallium garnet (Gd3Ga5O12) single crystal to environmentally unstable polycrystalline organo-halide perovskite thin films to perovskite based solar cells. A significant component of this work is real time monitoring of time sensitive perovskite thin film during the growth as well as post deposition degradation and demonstrated illustrative ideas of data analysis techniques for real time measurement. A commercially available single crystal Gd3Ga5O12 is taken as the first case. The structural and optical properties of Gd3Ga5O12 from infrared to ultraviolet (0.034 to 5.89 eV) was extracted. A simple ellipsometry model is applied to analyze the data from near infrared to ultraviolet (0.74 to 5.89 eV). Additional transmission measurement adds to the accuracy below the band gap region where the absorption coefficient is less than 1000 cm-1 and subsequently helps to detect sub band gap features near 3.95 to 5.06 eV. The band gap is identified in the ultraviolet (UV), while transverse and longitudinal optical phonon modes in the infrared (IR) region are detected. Analysis of IR spectra and quantifying these phonon modes was challenging because of nine absorption peaks present and a sufficiently large number of (>30) fitting parameters. Another case study of this research work is on (FASnI3)1-x(MAPbI3)x (x = 0.00, 0.20, 0.35, 0.40, 0.60, and 1.00) perovskite thin films formed by combining formamidinium tin iodide (FASnI3) and methylammonium lead iodide (MAPbI3). Those perovskite films are time sensitive (open full item for complete abstract)

Committee: Nikolas Podraza (Advisor); Robert Collins (Committee Member); Randall Ellingson (Committee Member); Song Cheng (Committee Member); Dean Giolando (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2017, Physics

Spectroscopic ellipsometry (SE) is a powerful tool to characterize multilayered thin films, providing structural parameters and materials optical properties over a wide spectral range. Further analyses of these optical properties can provide additional information of interest on the physical and chemical properties of materials. In-situ real time SE (RTSE) combines high surface sensitivity with fast data acquisition and non-destructive probing, thus lends insights into the dynamics of film growth. In this dissertation, the methods of SE have been applied to investigate the growth and properties of material components used in the CIGS thin film photovoltaic technology. Examples of RTSE data collection and analyses are demonstrated for the growth of selenium (Se), molybdenum diselenide (MoSe2) and copper selenide (Cu2-xSe), used in CIGS technology which can then be applied in complete analysis of three-stage CIGS deposition by co-evaporation. Thin film Mo deposited by sputtering is the most widely used back contact for solar cells using CIGS absorbers. In this study, in-situ and real time characterization have been utilized in order to investigate the growth as well as the structural, optical, and electronic properties of Mo thin films deposited by DC magnetron sputtering at different substrate temperatures. In these studies, the surface roughness on the Mo is observed to decrease with increasing substrate temperature. The growth rate, nucleation behavior, evolution of surface roughness and development of void structures in Mo show strong variations with deposition temperature. In depth analyses of (e1, e2) provide consistent estimates of void fraction, excited carrier mean free path, group speeds of excited carriers and intrinsic stress in the films. Complementary ex-situ characterization of the as deposited Mo films included XRD, resistivity measurements by four-point-probe, SEM, and profilometry. This dissertation describes the research performed on the (In (open full item for complete abstract)

Committee: Robert Collins (Committee Chair); Nikolas J. Podraza (Committee Member); Bo Gao (Committee Member); Jacques G. Amar (Committee Member); Dean M. Giolando (Committee Member) Subjects: Materials Science; Physics; Solid State Physics

Doctor of Philosophy, University of Toledo, 2017, Electrical Engineering

As a result of recent advances in photovoltaics technology, renewable energy generated by the sun has become a viable alternative to traditional sources of energy. Photovoltaic modules composed of thin film solar cell devices offer several advantages over standard wafer based silicon modules, including high throughput automated production and reduced materials usage and cost. Spectroscopic ellipsometry (SE) is an important measurement tool capable of aiding high throughput manufacturing processes for thin films. SE is a non-destructive measurement technique well suited to measure and track parameters critical to photovoltaic device performance such as layer thicknesses, as well as optical and electrical properties of the layer components. This dissertation seeks to extend the application of SE via an expanded-beam method to large area photovoltaic modules while retaining the high measurement speeds of single spot measurements with collimated beams. The transition of ellipsometers as laboratory instruments to ones suitable for high throughput manufacturing lines poses unique challenges. The construction of a rotating compensator ellipsometer suitable for industrial applications is addressed with an emphasis on measurement speed. Schemes are evaluated to correct SE data for the inherent misalignments present in large area measurements of full-scale panels. In particular, consideration is given to the problems of oscillations due to compensator misalignment, effects of glass stress and overlap of reflected beams in through-the-glass measurements, and off-plane corrections due to large area substrate curvature. Expanded-beam SE was developed and applied for in situ, high-speed imaging/mapping analysis of spatial uniformity over large area multilayer coated substrates used in roll-to-roll thin film photovoltaics (PV). Slower speed instrumentation available for such analysis applies a 1D detector array for spectroscopic mapping andinvolves width-wise translation of (open full item for complete abstract)

Committee: Robert Collins (Advisor); Anthony Johnson (Committee Member); Daniel Georgiev (Committee Member); Mohammed Niamat (Committee Member); Nikolas Podraza (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, University of Toledo, 2017, Physics

This dissertation describes a collection of studies that demonstrate and expand the many configurations in which spectroscopic ellipsometry (SE) can be applied to material characterization, primarily for thin films. The materials investigated each have relevance to photovoltaics, but the methods described herein can be applicable to the study of materials used in virtually any application. In aggregate, the measurement and data modeling techniques represent a broad set of tools that can be used to study the optoelectronic and structural characteristics of amorphous, polycrystalline, nanostructured, or inhomogeneous layers within thin film solar cell devices. The capabilities for SE to determine properties of a sample of interest well beyond simple optical response functions are demonstrated. In particular, SE is used in a real-time, in situ configuration where a series of measurements are taken continually during the deposition of all layers in complete hydrogenated amorphous silicon (a-Si:H) solar cells. Thus, the application of real-time SE (RTSE) to the entire process of solar cell fabrication is realized. The optical response and thicknesses of each layer are obtained and are used to interpret variation in the measured electrical performance between different devices. The SE-derived results are then used as inputs to a simulation of the expected current generated by the devices, the results of which were successful in identifying damage to a transparent conducting layer resulting from exposure to plasma during sample fabrication as the source of performance losses. The study of full a-Si:H-based solar cells revealed the presence of subtle optical property gradients within individual layers. The ability to characterize slight inhomogeneity using RTSE was further developed using measurements collected for a-Si:H films deposited under various conditions and on various substrates. In particular, this work systematically examines a range of modeling configurations (open full item for complete abstract)

Committee: Nikolas Podraza Dr. (Advisor); Collins Robert Dr. (Committee Member); Yan Yanfa Dr. (Committee Member); Gao Bo Dr. (Committee Member); Hofmann Tino Dr. (Committee Member) Subjects: Alternative Energy; Meteorology; Optics; Physics

Master of Science, University of Akron, 2016, Polymer Engineering

Weak polyelectrolyte multilayers (PEMs) fabricated by the Layer by Layer (LBL) assembling technique showed special swollen and contraction behaviors when they came in contact with water and organic liquids. The contraction responses of PEM were found to be dependent on the solvent selection. The correlation between the degrees of the films contraction and the solvent type needed be explored. Therefore, in this study, we utilized solubility parameters to discuss the responses for branched poly (ethylene imine)/poly (acrylic acid) (BPEI/PAA) multilayers when soaked in a variety of solvent liquids. When immersed in organic solvents, film dehydrated and contraction also caused mechanical property changes for BPEI/PAA films. The film's thickness was the best predictor for determining how a film swelled in water or contracted in organic liquid. The hydrogen bonding ability of the solvents played an important role in determining the degree of film contraction in most cases, for these solvents, it did so when increasing the temperature of the measurement corresponding to reducing the strength of the hydrogen bonding, and decreasing the ability to dehydrate the films as well. However, some solvents did not follow the linear trend with the strength of hydrogen bonding; in these, a stronger correlation was observed between contraction degrees and dielectric constants, showing that traditional solvent quality discussions and electrostatics were significant to understanding the contraction behavior of PEMs in organic solvents. Besides the PEM system, the swelling behavior of poly(n-propyl methacrylate), PPMA, films in water were also measured in-situ using spectroscopic ellipsometry. Two different end groups grafted poly (n-propyl methacrylate)s with similar high molecular weight were successfully synthesized by the reversible addition-fragmentation chain-transfer polymerization (RAFT) synthesis method. The end group of the two PPMAs impacted the swelling behavior over the te (open full item for complete abstract)

Committee: Nicole Zacharia Dr. (Advisor); Bryan Vogt Dr. (Advisor); Alamgir Karim Dr. (Committee Chair) Subjects: Polymer Chemistry

Doctor of Philosophy, University of Toledo, 2015, Physics

Spectroscopic ellipsometry (SE) in the mid-infrared to ultraviolet range has been implemented in order to develop and evaluate optimization procedures for CdTe solar cells at the different stages of fabrication. In this dissertation research, real time SE (RT-SE) has been applied during the fabrication of the as-deposited CdS/CdTe solar cell. Two areas of background research were addressed before undertaking the challenging RT-SE analysis procedures. First, optical functions were parameterized versus temperature for the glass substrate and its overlayers, including three different SnO2 layers. This database has applications not only for RT-SE analysis but also for on-line monitoring of the coated glass itself at elevated temperature. Second, post-deposition modifications of substrate have been studied by infrared spectroscopic ellipsometry (IR-SE) prior to the RT-SE analysis in order to evaluate the need for such modification in the analysis. With support from these background studies, RT-SE has been implemented in analyses of the evolution of the thin film structural properties during sputter deposition of polycrystalline CdS/CdTe solar cells on the transparent conducting oxide (TCO) coated glass substrates. The real time optical spectra collected during CdS/CdTe deposition were analyzed using the optical property database for all substrate components as a function of measurement temperature. RT-SE enables characterization of the filling process of the surface roughness modulations on the top-most SnO2 substrate layer, commonly referred to as the high resistivity transparent (HRT) layer. In this filling process, the optical properties of this surface layer are modified in accordance with an effective medium theory. In addition to providing information on interface formation to the substrate during film growth, RT-SE also provides information on the bulk layer CdS growth, its surface roughness evolution, as well as overlying CdTe interface formation and bulk layer g (open full item for complete abstract)

Committee: Robert Collins Dr. (Committee Chair); Nikolas Podraza Dr. (Committee Member); Yanfa Yan Dr. (Committee Member); Sanjay Khare Dr. (Committee Member); Stephen O'Leary Dr. (Committee Member) Subjects: Physics

Doctor of Philosophy, University of Toledo, 2015, Physics

Many modern optical and electronic devices, including photovoltaic devices, consist of multilayered thin film structures. Spectroscopic ellipsometry (SE) is a critically important characterization technique for such multilayers. SE can be applied to measure key parameters related to the structural, optical, and electrical properties of the components of multilayers with high accuracy and precision. One of the key advantages of this non-destructive technique is its capability of monitoring the growth dynamics of thin films in-situ and in real time with monolayer level precision. In this dissertation, the techniques of SE have been applied to study the component layer materials and structures used as back-reflectors and as the transparent contact layers in thin film photovoltaic technologies, including hydrogenated silicon (Si:H), copper indium-gallium diselenide (CIGS), and cadmium telluride (CdTe). The component layer materials, including silver and both intrinsic and doped zinc oxide, are fabricated on crystalline silicon and glass substrates using magnetron sputtering techniques. These thin films are measured in-situ and in real time as well as ex-situ by spectroscopic ellipsometry in order to extract parameters related to the structural properties, such as bulk layer thickness and surface roughness layer thickness and their time evolution, the latter information specific to real time measurements. The index of refraction and extinction coefficient or complex dielectric function of a single unknown layer can also be obtained from the measurement versus photon energy. Applying analytical expressions for these optical properties versus photon energy, parameters that describe electronic transport, such as electrical resistivity and electron scattering time, can be extracted. The SE technique is also performed as the sample is heated in order to derive the effects of annealing on the optical properties and derived electrical transport parameters, as well as (open full item for complete abstract)

Committee: Robert Collins (Advisor) Subjects: Optics; Physics; Solid State Physics

Doctor of Philosophy (Ph.D.), University of Dayton, 2015, Materials Engineering

This work details two specific research thrusts exploring the deposition and characterization of mixed valent oxide systems. The first of these thrusts investigated the effect of the oxygen content, during reactive sputter deposition, on the optical, chemical, and structural properties of oxides of molybdenum, germanium, and rhenium. Exploration of the Mo-O system was conducted using a technique known as modulated pulse power magnetron sputtering (MPPMS), while the Ge-O and Re-O systems were deposited via direct current magnetron sputtering (DCMS). Films deposited under poisoned mode conditions were shown to be highly transparent with refractive index (n) values of n550=1.60 for GeO2, and n550=1.97 for MoO3, similar to values reported for bulk constituents. The Re-O system, unlike Ge-O and Mo-O, displayed a significantly high sensitivity to ambient moisture. Chemical analysis via XPS indicated the presence of instability as a result of the moisture induced decomposition of Re2O7 into HReO4, and catalytic disproportionation of Re2O3 into Re and hydrous ReO2. The second research thrust within this project was focused on the deposition of three component mixed oxide systems with multiple valence states. This effort, which utilized the results from individual material depositions mentioned previously, required the use of stable and thermodynamically compatible material systems, namely Mo-O and Ge-O (ΔfHo(MoO2)= -588 kJ/mol and ΔfHo(GeO2)= -580 kJ/mol). Note that Re-O was not explored as part of the ternary deposition effort due to the aforementioned chemical instability. To achieve the goal of depositing mixed valent thin films with tailorable optical absorption, an industrially scalable co-deposition method was devised in order to deposit molybdenum cations within a dielectric GeO2 matrix. The high power densities associated with the MPPMS process were systematically varied in order to control the oxygen partial pressure via gettering, allowing for control over the (open full item for complete abstract)

Committee: P. Terrence Murray Ph.D. (Committee Chair); Dean R. Evans Ph.D. (Advisor); John T. Grant Ph.D. (Committee Member); Daniel P. Kramer Ph.D. (Committee Member); Andrew M. Sarangan Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, University of Toledo, 2014, Physics

The demand for clean and renewable energy sources in recent years has motivated research on the development of low cost, thin film photovoltaic devices. As a consequence, tools for the investigation and characterization of thin film photovoltaic component materials and devices, which can be implemented in real time as well as under in-line and off-line measurement conditions, are becoming increasingly important. Real time spectroscopic ellipsometry (RTSE) and ex-situ mapping spectroscopic ellipsometry (SE) are powerful characterization tools suitable for applications in the optimization of device performance and the evaluation of thin film photovoltaics technology scale-up from dot cell sizes in research laboratories to full module sizes in factories. These non-destructive optical probes implement multichannel spectroscopic detection for achieving high measurement speed, while simultaneously yielding high precision light-matter interaction parameters. The interaction parameters can be analyzed to obtain layer thicknesses as well as their optical properties from which material properties such as composition can be determined. The layer thicknesses and their optical properties in turn provide insights into the fraction of incident light absorbed in the active layer of the solar cell and also provide a basis for short-circuit current optimization through optical simulations. In this dissertation research, Cu(In,Ga)Se2 films with different Ga contents have been prepared by a one stage co-evaporation process. These films have been studied by spectroscopic ellipsometry (RTSE) in real time during their deposition, which has been performed at high temperature (570oC). After cooling the films to room temperature, in-situ SE measurements were undertaken in order to extract the dielectric functions of the thin film materials. An extended parameterization was established through the fitting of these dielectric functions to analytical functions, followed by the development (open full item for complete abstract)

Committee: Robert W. Collins (Committee Chair); Song Cheng (Committee Member); Randall Ellingson (Committee Member); Nikolas Podraza (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2007, Arts and Sciences : Chemistry

Thin ion-exchange polymer films (henceforth polyelectrolytes) have recently acquired substantial research interest. This is due in large part to the great promise demonstrated by the effective uptake and large preconcentration ratios of the ions from a liquid environment. The nanostructured porous film is an essential element of the spectroelectrochemical sensors. One fundamental aspect of the research involves molecular mass transport at the liquid/solid interface and associated film dynamics. These were studied using spectroscopic ellipsometry and quartz crystal microgravimetry and a good overall agreement between methods has been confirmed. The methodology for diffusion modeling was developed and demonstrated using several polyelectrolyte film materials. Three different theoretical models that account for non-ideal behaviors were employed to solve for the correct mechanism. Slow polymer relaxations coupled with ideal Fickian diffusion caused anomalies. The mass transport of model analytes was characterized by relatively slow diffusion constants (D = 10-12 – 10-14 cm2/s) and relaxation rate constants (kR = 10-4 – 10-5 s-1). Anomalous diffusion in poly(vinyl alcohol)-poly(acrylic acid) (PVA–PAA) composite thin films and the structure-property relationship were studied in detail. The results of in situ complexation between Fe2+ and ligand (2,2'-bipyridine) in thin Nafion films gave important insight into the set of chemical reactions for aqueous iron detection. The last chapter focuses on dynamics of a novel polyelectrolyte film material for chemical sensing, namely, partially sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SSEBS). The physico-chemical phenomenon of molecular aggregation was studied using rhodamine 6G laser dye and the SSEBS film material. It was demonstrated for the first time how spectroscopic ellipsometry could be effectively used in studying such a subtle process in thin films. The two thesis appendices are devoted to (open full item for complete abstract)

Committee: Dr. Carl Seliskar (Committee Chair); Dr. William Heineman (Other); Dr. Bruce Ault (Other) Subjects:

PhD, University of Cincinnati, 2003, Arts and Sciences : Chemistry

This thesis describes ellipsometric studies of chemically selective films to characterize certain aspects of spectroelectrochemical sensors. For these intentions back-side ellipsometric configuration was adopted that enables in situ studies of coatings that contact solution. Two slightly different flow cells that enable such in situ interrogations were developed. The method was first demonstrated by acid etching of ITO film on glass substrate. Continuous and uniform film's thickness decrease was observed in this process. A characterization procedure of ITO films on 1737 F glass substrate is outlined and optical constant determination of this material is presented. Several sensor film formulations were characterized too. A partition process of a chromophore (Ru(bpy) 32+) into polymer film (Nafion) was studied and it was determined how this film's properties (optical constants and thickness) are affected by incorporation of the chromophore. Three modes of chromophore partitioning were modeled and ellipsometric data were used to distinguish between these three modes. As has been noted before, some film formulations require long equilibration with 0.1 M aqueous KNO3 solution for optimal performance. This observation suggested analysis of this equilibration process. Several different film dynamics in soaking solution were studied ellipsometricly. Film behavior ranged from no observable changes to eventual complete and irreversible collapse. Comparisons of experiments conducted ellipsometricly and electrochemically to elucidate this equilibration was presented.

Committee: Dr. Carl J. Seliskar (Advisor); Dr. William R. Heineman (Other) Subjects: Chemistry, Analytical

Doctor of Philosophy, University of Toledo, 2012, Physics

As the demand for clean, renewable energy sources increases, the development of high efficiency, low cost photovoltaic devices from thin films becomes increasingly important. Spectroscopic ellipsometry is a promising tool for the characterization and investigation of thin film photovoltaic devices and the component materials from which they are made. This tool can be applied either in-situ and in real-time using high speed multichannel instruments, or ex-situ using slower wavelength-by-wavelength scanning instruments. Spectroscopic ellipsometry is promising for use in thin film photovoltaics because it provides the thicknesses of the individual layers and their optical properties, which in turn provide insights into the light collection required for photocurrent generation. Advanced forms of ellipsometry include (i) ex-situ mapping spectroscopic ellipsometry with a multichannel ellipsometer, which provides information on thickness and optical property non-uniformities over large areas of a coated substrate, and (ii) real-time spectroscopic ellipsometry with similar high speed instrumentation, which provides information on thin film nucleation, coalescence, and growth, as well as changes in the structure of the material over time. Both advanced forms of ellipsometry have been applied in this Dissertation to analyze thin films with applications in photovoltaics. In this Dissertation research, ex-situ mapping ellipsometry has been used to study Au nanoparticle thin films. These films are useful because they can be integrated into solar cells to promote light trapping within the absorber layers, and hence, increase the overall efficiency of the cells. Studying these films with mapping spectroscopic ellipsometry provides a means for determining thickness uniformity over large areas of the sample for scale-up of the deposition processes. The uniformity of other parameters of the Au nanoparticle films such as the plasmon resonance band energy and its broadening are also (open full item for complete abstract)

Committee: Dr. Robert Collins (Advisor); Dr. Karen Bjorkman (Committee Member); Dr. Jacques Amar (Committee Member); Dr. Nikolas Podraza (Committee Member); Dr. Terry Bigioni (Committee Member) Subjects: Energy; Physics